In the field of dental implantology in particular, implants have in practice proven useful in which the bone contact surface is made of metals, in particular of titanium or of titanium alloys. In the hydroxylated and hydrophilic state in particular, such materials can provide excellent osseointegration (see, for example, WO 00/44305).
In the area of the soft tissue, by contrast, such materials are associated with disadvantages. On the one hand, the darkish metal implant shows through the soft tissue, as a result of which the visual impression created by such treatment may be marred. In addition, reports have described how it is possible with other materials, for example ceramics, to achieve an improved tissue reaction in the soft tissue area (see German utility model DE-U1-298 20 971, for example).
It is thus known, from a large number of documents, to equip a metal implant with a ceramic sleeve arranged in the area of the soft tissue contact surface (see, for example, documents DE-U1-298 20 971, U.S. Pat. No. 5,152,687, FR-B3-2 788 963, GB-A-2 139 095). Ceramic sleeves of this kind are, for example, sintered or adhesively bonded onto the implant. As a result, however, a number of problems arise which have not as yet been resolved. On the one hand, using a ceramic sleeve as a separate structural part almost unavoidably results in a micro-gap, which in particular can cause bacterial contamination.
By contrast, when such a sleeve is fitted by sintering, such high temperatures generally have to be used that the metallic base body of the implant at least partially oxidizes. However, this has a disadvantageous effect on osseointegration. Moreover, different coefficients of thermal expansion of the materials involved can result, particularly during cooling, in the development of microstresses, and these can lead to hairline fractures and cracks.
It is known, for example from document EP-B1-0 211 676, to provide titanium implants with a ceramic coating of the entire implant body. It has likewise been proposed to provide the bone contact surface of an implant with a biocompatible coating, for example of hydroxyapatite or calcium phosphate (e.g. U.S. Pat. No. 5,478,237). However, by doing so, the proven advantageous properties of titanium in respect of osseointegration are no longer available.
Many methods of coating a substrate are known:
Coatings can be applied by means of CVD or PVD (chemical vapour deposition or physical vapour deposition). A disadvantage of these methods is that adhesion to the surface is generally not optimal and the coating may be brittle. Moreover, the colour of a coating obtainable in this way cannot generally be influenced to the desired extent by its thickness.
Coatings with silicate glass and with silicate-glass-modified ceramics are also known, for example from document WO 01/74730. A disadvantage of these is, once again, the possibility of microstresses occurring as a result of the firing process at high temperatures and the at least partial oxidation of the titanium surface.
It is also known to coat metals such as aluminium, titanium or tantalum by means of anodic oxidation under spark discharge. Document DE-C1-43 03 575 describes the production of implants coated with apatite. Document DD-A1-246 028 describes a metal implant which, in the bone contact area, is coated with a calcium phosphate ceramic. Here again, however, the proven advantageous properties of titanium in particular with respect to osseointegration are no longer available.